A SEAL is a highly-trained special forces soldier of the US Navy. Part of a group of 2, 4, 8, or no higher than 16 other SEAL's, Navy SEAL's are pretty much the modern-day ninja; they are trained to meld in with the scenery and attack without being noticed. Their bodies are trained and pushed beyond almost human-limits.

"Not everyone can be a SEAL. In fact, you'd have to be superhuman to be one! If you really want to be a SEAL, you really have to want it."
-me

"The only easy day was yesterday!"
-SEAL Motto

"If you're called to duty but you're not ready, then you've failed."
-SEAL Moral from an officer

a substance, strip of material, used to fill a crack so that air, liquid, etc. cannot get in or out.
A piece of wax that is placed across the opening of letters.
A piece of metal, a ring with a design on it, used for stamping a seal.

1. A seal is basically a girl/guy (or even an item) that you can't have in the sense that she/he/it can never be yours.

Being a seal is relative since your seal probably hooks with people (who you probably hate for this fact) and is therefore not a seal to them.

This definition of seal comes from the person's likeness to the animal: the fact that they're so cute/hot/desirable that you want them but you also simultaneously want to club over the head (like with baby seals) because you can never have them.

2. Also, this can be used as a suffix or modifier to names to disguise the fact that you are talking about someone in the same room. Usually that person is a seal by the above definition as well before 'seal' is used to modify their name.

1.
Guy 1 (frustrated): "Man! There are so many seals in this bar tonight!!"

Guy 2: "Yeah I know... I just wanna club them over the head so no one else can have them..."

OR

Guy 1: "Who's that cute girl by the bar?"
Guy 2: " Hmm I don't know her name but she's definitely a baby seal."

***

2.
Elsa = Anagram Seal (because the name is an anagram of the word seal)